Two Months - Calendar Months or 60 Days?

I am visiting Britain from 27th April to June 6th, which is less than 60 days, but could be seen as three months? I don't want my pass to expire while I still have days left on it.

If 'two months' means calendar months, or if I need more than 15 days travel in two months, can I buy more than one BritRail Pass?
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